DEC PDP-8
2024 Restoration

22-May-24

We did some more investigating this afternoon. Doug reminded us that there are early and late versions of the PDP-8 Maintenance Manual, and there are some design changes in the later version. Since this is S/N 13 it is probably a very early version.


There are a bunch of empty slots on the Memory side of the chassis because the #183 Memory Extension Control and #188 Memory Parity options are not installed.


The FlipChips in slots: 

MA30 through MA25 match the F-87 manual.

MB31 through MB25 match the F-87 manual.

MC31 through MC06 match the F-87 manual except that it has a B684 Two Bus Drivers FlipChip in slot MC19 instead of a B602.

MD30 through MD06 match the F-87 manual except that it has G203B Memory Selectors instead of G209. It looks like a G209 is an upgraded replacement for a G203.

ME28 through ME14 match the F-87 manual.

MF28 through MF15 match the F-87 manual.

There is an R405 Crystal Clock in slot MF29 and an R303 Integrating One-Shot in slot ME29 that are not in the F-87 manual. There are a bunch of white wires going to the R303, and one white wire going to Pin D of the M405. The R405 schematic says that pin D is the clock output. There are three wires that go from the slot ME29 to the Processor side. There are many white wires on the processor side. I will need to make a schematic all of the white wires and additional FlipChips so we can determine what this modification does.

The back of the I/O and Memory Chassis

The back of the Processor Chassis

24-May-24

The empty slots in the Processor side of the chassis are for the #189 A-D Converter, KR01 Power Interrupt, and #681 Data Line Interface options that are not installed.

The FlipChips in slots PA36 through PA01 match the F-87 manual, except that there is an S111 Diode Gate in slots PA22, PA24, PA25, PA27, PA30, and PA31 instead of a lower power R111, and an R302 Delay in slot PA36 instead of a W501 Schmitt Trigger.

The FlipChips in slots PB36 through PB01 match the F-87 manual, except that they are High-Power S versions of the FlipChips in slots PB19 through PB34 instead of the lower power R FlipChips. Slot PB35 holds an R602 Pulse Amplifier instead of an R405 Crystal Clock. Maybe the R405 was moved to slot MF29?
The FlipChips in slots PC36 through PC01 match the F-87 manual, except that many of the Lower-Power R versions have been replaced with High-Power S versions.
The FlipChips in slots PD36 through PD01 match the F-87 manual, except that many of the Lower-Power R versions have been replaced with High-Power S versions.
The FlipChips in slots PE36 through PE01 match the F-87 manual, except that many of the Lower-Power R versions have been replaced with High-Power S versions.
The FlipChips in slots PF36 through PF01 match the F-87 manual, except that many of the Lower-Power R versions have been replaced with High-Power S versions.

25-May-24

We removed all 12 front panel switches that had broken pivots. We need to 3D print a drilling fixture, drill out the broken pivots, and replace the pivots with 1/16" or 0.0625" acrylic rod. We reassembled the front panel with the good switches in the far right positions.

We took better pictures of the Processor clock modifications. We need to reverse engineer this clock circuit.

29-May-24

We disconnected the power harness connecting the backplanes to the power supply and removed the power supply. That will allow for a closer inspection for damaged components. allow better access so we can reform the capacitors, and make it easier to test.

1-Jun-24

We think that this system has not been powered for about 50 years, so we removed the H708 power supply from the chassis. We inspected everything for damage and leakage, reformed the capacitors. and checked the diodes and transistors. We connected it to an AC source through a Variac, and slowly ramped up the AC voltage over about an hour while monitoring the 6x output voltages. At about 45% voltage the POWER OK relay chattered and closed. At about 65% voltage all of the output voltages looked reasonable. The Margin Supply actually adjusted as expected. We reinstalled the power supply and turned the system on. Five of the six fans spun up and some lights on the front panel lit. All of the control switches on the front panel actually do something. The LOAD ADDRESS switch looks like it is actually loading the switch settings into the PC.

We printed Doug's front panel switch handle drill fixture. This week I will drill out the broken handle pivot pins. I bought some 1/16" acrylic rod on eBay to use for the replacement pivots.

Next week we will replace the bulbs on the front panel that are burned out. We have just a few spare original bulbs. We will also take detailed notes on the control switch behavior and start debugging the system. 

It shows some signs of life

5-Jun-24

We found a DEC cabinet in the warehouse that was 40" high, strong enough to hold the 220 pounds of PDP-8 on top, and had casters. It is a little higher that what would be perfect, but it should work OK. Our musclebound neighbors were kind to do most of the lifting to put the PDP-8 on top of the cabinet.

There are sure to be some burned out bulbs in the front panel. The original bulbs have really tiny leads coming out of the glass bulb and are really fragile. I bought some CM2187 bulbs from DigiKey for replacements. The bulbs and 1/16" acrylic rod for the switch pivots have not arrived yet, so maybe next week we can start the repairs on the front panel.

We resoldered the broken wire on the back of the PANEL LOCK switch. It doesn't seem to have made a difference.

Light and Switch behavior:

The back of the PANEL LOCK switch

12-Jun-24

The 1/16" Acrylic rod for the switch pivots arrived. We did a test fit in the metal switch brackets before we drilled the switches. It fits just fine. Doug said that the Acrylic rod has less clearance than the original molded plastic switches so the don't wiggle as much.

We decided to debug the DEP and EXAM functions because we can't do anything without working memory. We started with the Timing, Keys, Switches, and RUN control on page BS-D-8P-0-9 of the schematics. We looked at the KEY EX+DEP in section C3 on pin N of the R107 inverter in slot PB30. The signal is normally at -4VDC and goes to ground when either EXAM or DEP is pressed. That means that both switches, the wiring, and parts of the R111 in slot PB31 and the R107 in slot PB30 are working OK. We looked at the KEY ST+EX+DP signal on pin F of the R107 in slot PB33. The signal is normally at -4VDC and goes to ground when START, EXAM, or DEP is pressed.

Time to look at the SP0 in section A3 of the same page, and SP1, SP2, and SP3 signals in section A6 of the same page. We expected to find a W501 Schmitt Trigger in slot PA36, but instead found an R302 Delay. The wires to the R302 are white, so they have been changed after the factory built the system. We looked at the SP0 signal on pin E of the R302 in slot PA35. We see a 2uS negative going pulse when START or LOAD ADDRESS is pressed, but not when EXAM or DEP is pressed, and only once when CONT is pressed. We looked at the signal on pin J of the S111 in slot PB31. It goes low when the START, LOAD ADDRESS, EXAM, DEP, and CONT switches are operated. 

This is the area where the processor wiring has been modified. We will need to reverse engineer the new wiring to determine why the EXAM and DEP functions are not working.

Pin H & J on the S111 in slot BP31 has the KEY ST+EX+DP+LOAD ADDRESS+CONT signal. This signal goes low when any of these keys are pressed. The signal is not debounced and is a logic level. The white wire from PB31 Pin H goes into the power harness and into the cover that holds the Margin Power switch. We disassemble the cover and found that the wire goes to the POWER OK signal. I guess that would inhibit operating the console if the power was not OK. Pins PB31-H and PB31-J are wired together to provide a pull-down on the signal.

We looked at CLOCK signal on Pin D of the R405 in slot MF29. This FlipChip was originally in slot PB35. We see a 1.33 MHz square wave going to ground. MF29-D is wired to the R602 Pulse Amplifier PB35-H which is gated by PB35-J. The output should be on pin PB35-K. There is an additional input on PB35-E that is gated by PB35-F. We see a 1.33 MHz square wave going to ground on pin PB35-K. PB35-K is wired to the S202 Dial flipflop in slot PB34-U. Pin PB34-U is a gated input to the (1) side of the RUN flipflop. This is the signal on the RUN flipflop,

15-Jun-24

For some unknown reason the EXAM and DEP switches are now incrementing the PROGRAM COUNTER and MEMORY ADDRESS when the keys are pressed.

19-Jun-24

We spent the afternoon reverse engineering the modifications to the Clock and Timing circuitry. The R405 clock board was moved from PB35 to MF29, and an was R602 Pulse Amplifier installed in PB35. An R303 Integrating One-Shot was added in slot ME29.

22-Jun-24

We started making a schematic that shows the modifications to the Clock and Timing circuits. We are not sure why the modifications were done, and are considering reverting to the original design.

26-Jun-24

We drilled out the broken switch pivots using a #52 (0.063") drill and Doug's drill fixture that we 3D printed.

We cut lengths of 1/15" Acrylic rod to make replacement pivots and glued them in place. It is really fiddly work, but the result is better than the original pivots.

To Do: